This invention relates to novel methods of synthesizing acid functional fluoropolymers by dehydrofluorination and nucleophilic addition of an acidifiable group, followed by acidification of the acidifiable group to obtain acid functionality. This invention also relates to the polymers and membranes made or modified according to such methods, particularly for use as ion conducting membranes.
Electrochemical devices, including proton exchange membrane fuel cells, electrolyzers, chlor-alkali separation membranes, and the like, are typically constructed from a unit referred to as a membrane electrode assembly (MEA). Such MEA""s comprise one or more electrode portions, which include a catalytic electrode material such as Pt or Pd, in contact with an ion conductive membrane. Ion conductive membranes (ICMs) are used in electrochemical cells as solid electrolytes. In a typical electrochemical cell, an ICM is in contact with cathode and anode electrodes, and transports ions such as protons that are formed at the anode to the cathode, allowing a current of electrons to flow in an external circuit connecting the electrodes.
In a typical hydrogen/oxygen fuel cell, the ions to be conducted by the membrane are protons. Importantly, ICMs do not conduct electrons/electricity, since this would render the fuel cell useless, and they must be essentially impermeable to fuel gasses, such as hydrogen and oxygen. Any leakage of the gasses employed in the reaction across the MEA results in waste of the reactants and inefficiency of the cell. For that reason, the ion exchange membrane must have low or no permeability to the gasses employed in the reaction.
ICMs also find use in chlor-alkali cells wherein brine mixtures are separated to form chlorine gas and sodium hydroxide. The membrane selectively transports sodium ions while rejecting chloride ions. Such membranes may also be useful in batteries and electrochemical storage cells, particularly membranes that transport lithium ions. ICMs also can be useful for applications such as diffusion dialysis, electrodialysis, and pervaporization and vapor permeation separations. While most ICMs transport cations or protons, membranes that are transportive to anions such as OHxe2x88x92are known and commercially available.
Commercially-available ICMs are not entirely satisfactory in meeting the performance demands of fuel cells. For example, Nafion(trademark) membranes (DuPont Chemicals, Inc., Wilmington, Del.), which are perfluorocarbon materials having pendent sulfonate groups, are considered expensive and structurally weak when wet. Nafion membranes are not generally available at thicknesses of less than 50 xcexcm. While Nafion membranes with lower equivalent weight can be used to obtain lower ionic resistance, lower equivalent weight membranes are structurally weaker and thus require reinforcement.
The search for new acid-functional fluoropolymers has been impeded by the difficulty inherent in copolymerizing acid-functional fluoromonomers with tetrafluoroethylene or other suitable perfluoro comonomers.
U.S. Pat. No. 4,894,410 and U.S. Pat. No. 4,956,419 (3M) disclose the manufacture of fluoropolymer membranes having various functional groups appended through thio linkages.
U.S. Pat. No. 5,395,886 (Dow Corning) discloses a method of modifying partially-fluorinated hydrocarbon polymers to provide latent reactive substituents and polymers crosslinked by means of those substituents. The latent reactive substituents are appended by nucleophilic addition subsequent or concurrent to dehydrofluorination of the polymer. The reference does not disclose a polymer membrane sufficiently substituted with acidic functions to function as an ion conducting membrane.
U.S. Pat. No. 5,656,386 (Paul Scherrer Institut) discloses fluoropolymer membranes having various functional groups appended by a radiation grafting method.
Briefly, the present invention provides a method of making an acid functional fluoropolymer by: a) dehydrofluorinating a starting fluoropolymer with a dehydrofluorinating agent to form an unsaturated fluoropolymer; b) adding an acidifiable nucleophilic functionalizing agent to a double bond of the unsaturated fluoropolymer; and c) acidifying the added acidifiable function.
In another aspect, the present invention provides acid functional fluorocarbon membranes for use as ion conducting membranes in electrochemical cells.
In another aspect, the present invention provides a method of making an ion conducting membrane (ICM) by: a) dehydrofluorinating a starting fluoropolymer with a dehydrofluorinating agent to form an unsaturated fluoropolymer; b) adding an acidifiable nucleophilic functionalizing agent to a double bond of the unsaturated fluoropolymer to form a fluoropolymer bearing an acidifiable function; c) forming the fluoropolymer bearing an acidifiable function into a membrane; and d) acidifying said acidifiable function to form an ICM.
In another aspect, the present invention provides acid functional fluoropolymers having pendent groups according to the formula: xe2x80x94Xxe2x80x94Arxe2x80x94An, wherein X is selected from O, S or NR, where R is selected from H and C1-C30 alkyl or aryl, which are optionally substituted, wherein Ar is a C6-C30 aromatic group, which is optionally substituted, wherein A is an acidic function or salt thereof, wherein a can be independently chosen to be 1, 2 or 3; and wherein said acid functional fluoropolymer is sufficiently acidified as to meet a condition selected from: a) the equivalent weight of the polymer is 5000 or less; and b) the proton conductivity of the polymer at 25xc2x0 C. is 0.01 Siemens per centimeter (S/cm) or higher. In addition, ion conducting membranes of such acid functional fluoropolymers are provided.
What has not been described in the art, and is provided by the present invention, is a method of providing acid functionalized fluoropolymer materials usable as ion conducting membranes, such as those used in electrolytic cells.
In this application,
xe2x80x9cacidifiablexe2x80x9d group, function or agent means either a) an acid-receiving group which is readily capable of substitution with an acid function, preferably by exposure to an acid, such as an aromatic group which may be acidified by treatment with sulfuric acid, or b) a proto-acid function which is capable of facile conversion to an acid, preferably by hydrolysis, such as a sulfonyl halide, but preferably a);
xe2x80x9cequivalent weightxe2x80x9d means the mass of an acidic material that contains one mole of acid functional groups; and
when used without reference to a particular substituent, xe2x80x9csubstitutedxe2x80x9d means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide fluoropolymer ion conducting membranes for use in an electrolytic cell. It is a further advantage to provide a simple synthetic route to such membranes.